A variety of surface light source devices that are used to illuminate a transmission type of liquid crystal displays or the like from their rear have been proposed and put into practical use. Such surface light source devices are broadly classified into the edge light type and the direct type. These types differ from each other in the mode of conversion of a non-surface light source to a surface light source.
For example, a surface light source device of the direct type is so constructed that light is introduced from its rear by cold-cathode tubes arranged in parallel. The cold-cathode tubes and a transmission type of display element such as an LCD (Liquid Crystal Display) panel are properly spaced from each other. In this space, there are placed a diffusing sheet and a combination of two or more sheets capable of converging light.
Such a conventional surface light source device is poor in the light-converging properties, although the number of required optical sheets is large. Thus, the LCD panel has been improved for compensating the light-converging properties, so that even light that is obliquely incident can produce an image of excellent qualities.
However, this improvement suffers reduction in optical efficiency, and complicates the structure of the LCD panel, which results in increase in cost.
In particular, the light intensity (luminance) tends to be non-uniform (luminance non-uniformity tends to occur) depending on the distance from the cold-cathode tubes (i.e., whether that point is close to a certain cold-cathode tube or to a space between the cold-cathode tubes arranged in parallel). A possible method of avoiding the non-uniformity is to space the cold-cathode tubes and the LCD panel sufficiently apart from each other. However, this method undesirably increases a thickness of the display. Another possible method of avoiding the non-uniformity is to intense diffusion or to control an amount of transmitting light. However, this method disadvantageously degrades the optical efficiency.
For example, JP-A-5-119703 and JP-A-11-242219 can be raised as background art documents. In the surface light source devices disclosed in these documents, uniformity of light intensity (luminance) is maintained by providing a light-shielding member (lighting curtain, light-shielding dot layer). However, this method is disadvantageous in that the optical efficiency is lowered, similar to the above-described method.
JP-A-2005-115372 discloses that, in place of the cold-cathode tubes, independent light-emitting diodes (LED) of point light sources of red, green, and blue, which are the three primary colors of light, are used as light-emitting sources.
However, the use of arranged LEDs of independently different luminescent colors may invite color non-uniformity, in addition to the luminance non-uniformity as described above.
Moreover, since an LED is substantially a point light source, the luminance non-uniformity is likely to occur in a two-dimensional direction, as compared with a cold-cathode tube.